In the prior art there are basically two types of bi-directional solid state switches which are commonly used. One is the triac, which is commonly used for switching AC (alternating current) power, and the other is the analog switch based on field effect transistors.
Triacs suffer from two major problems. The first is that once triggered, they remain in the on-state until the next zero crossing in the AC voltage. The other problem is that the device inherently has a voltage drop associated with it, which results in significant power dissipation.
Switches based on field effect transistors have been very successfully used for switching low-level analog signals, however, various technical difficulties have prevented their widespread acceptance for AC power switching applications.
Referring now to FIG. 1, it is demonstrated how a single NPN bipolar transistor together with a diode bridge can form a simple AC switch. The NPN bipolar transistor is only capable of switching on in one direction, however, the diode bridge changes the AC at the switch terminals into DC (direct current) going through the transistor.
It can be appreciated that such a switch can only work if the control signal to the transistor is in some way isolated from the AC power voltage. One method of doing this, which is well known in the prior art, illustrated in FIG. 2, is to use an isolation transformer. Isolation transformers do not work on DC voltages, however, if an AC voltage is provided to the transformer input, a diode bridge on the output of the transformer can convert it into a DC voltage signal suitable for driving the input of the bipolar transistor. Still, such a switch is far from ideal due to power dissipation in the diodes and the bipolar transistor. This is due to the inherent voltage drops in these devices. A bipolar transistor which is fully on, will have a voltage drop of at least 0.3 Volts, however this voltage drop can be as high as 1 V or even higher depending on the transistor. Diodes have an inherent voltage drop of approximately 0.7 V, and since the AC would have to flow through two diodes plus the transistor, a total voltage drop across the switch would be on the order of 2 Volts. This means that the power dissipation will be about 2 Watts for every Amp flowing through the switch.
Referring now to FIG. 3 it is shown how power dissipation can be greatly reduced through the use of field effect transistors, however, the problem of isolation between the AC voltage and the control signal remains. Since field effect transistors can conduct in both directions when they are in the on-state, there is no need for a diode bridge. However, field effect transistors will only block voltage in one direction when in the off-state. Therefore, if they are used in an AC switch (without a diode bridge), there must be two of them connected in series in opposite directions, either having a common source as shown in FIG. 3, or having a common drain as shown in FIG. 5. The prior art method of isolation presented in FIG. 3 suffers from a number of problems. One is the size, weight, and cost of the isolation transformer. Another is the complexity of the control circuit which must generate AC control signals to drive the isolation transformer. Finally, there is a loss of speed of response due to the frequency limitation of the transformer and the rectification circuit.
Referring now to FIG. 4, another approach to isolation is shown which is most commonly used in the prior art. In this instance the control signal drives a series of light emitting diodes in a photo-voltaic isolator. Photo-voltaic cells in the isolator receive the light energy from the light emitting diodes and convert it to control signals suitable for driving the field effect transistors. This approach eliminates the bulky expensive isolation transformer, and simplifies the circuit. The two major drawbacks of this approach are the cost of the photo-voltaic isolator, and its slow speed of response. Solid state relays based on this technology typically have response times in the millisecond range.
The idea of connecting two field effect transistors (FET) together in a common source configuration is known. Huth et al show just such a configuration in U.S. Pat. No. 3,532,899 issued Oct. 6, 1970, which is incorporated herein by reference, for the purpose of providing a solid state switch for switching analog signals. However, Huth et al do not give any details on how to drive the field effect transistor inputs while maintaining adequate separation between the control signal for the switch and the analog signals being switched.
McDonald in U.S. Pat. No. 4,611,123 issued Sep. 9, 1986, which is incorporated herein by reference, also proposes a solid state switch consisting of two field effect transistors in a common source arrangement, and proposes a method of driving the field effect transistor inputs. However, his field effect transistor signal input driver involves the use of optical isolation. Optical isolation is a valid technical solution, however it does have certain drawbacks. In particular, it makes the circuit more complicated, it adds to the cost, and it slows down the speed of the device.
Sorchych in U.S. Pat. No. 3,215,859 issued Nov. 2, 1965, which is incorporated herein by reference, proposes a method of controlling field effect transistors without the use of signal isolation. Power supply isolation is also not required by the method which he proposes. However, his circuit is only suitable for switching low level analog signals. Any AC voltage of 120 V would subject his bipolar transistors, used as drivers, to excessive emitter-base voltages which would damage the devices. This makes Sorchych's device unsuitable for use as a power switch in many typical applications.
Jaeschke in U.S. Pat. No. 4,480,201 issued Oct. 30, 1984, which is incorporated herein by reference, provides a more robust method of driving the main switch transistors, however he uses bipolar transistors instead of field effect transistors for the output stage, choosing rather to use field effect transistors to improve the performance of the bipolar transistors. The result is a circuit which is not particularly energy-efficient. Any AC power going through the switch must pass through one diode having a diode drop of 0.7 Volts and one power transistor having a voltage drop of at least 0.3 Volts resulting in an overall voltage drop of at least 1 V. This results in a power dissipation of at least one Watt per Amp.
Laughton in U.S. Pat. No. 4,591,734 issued May 27, 1986, which is incorporated herein by reference, demonstrates the use of insulated gate bipolar transistors (IGBT) for the output of his AC switch. In addition to the problem of high power dissipation resulting from the voltage drop across the insulated gate bipolar transistors and diodes in the circuit, this circuit has limited use because Laughton grounds the emitters of the insulated gate bipolar transistors. This eliminates the need to isolate the signal input, however, this circuit can only be used where the AC voltage is isolated. In most cases this would require an expensive power isolation transformer, which would of course be more costly than a signal isolation transformer.
Chang et al in U.S. Pat. No. 4,742,380 issued May 3, 1988, which is incorporated herein by reference, show an AC switch which makes use of bipolar transistors, field effect transistors and thyristors. For this circuit to work properly, the emitter of the PNP transistor would need to be higher than the AC voltage. The result of this is the need for a high voltage DC power supply. Furthermore, this would not be a particularly energy efficient switch due to resistors in series with the field effect transistors, and the voltage drops across the thyristors. In addition to these drawbacks the control would be limited, because once triggered, the thyristors would remain on until the next zero crossing in the AC voltage.
Janutka in U.S. Pat. No. 4,477,742 issued Oct. 16, 1984, which is incorporated herein by reference, shows an AC switch with a common drain configuration. In this circuit the gates of the two field effect transistors are tied together. The result of this is that the voltage capability of the switch is limited by the gate-source breakdown voltages of the field effect transistors which is typically 20 Volts. This makes the circuit unsuitable for typical power applications which require substantially higher voltages.
Nadd in U.S. Pat. No. 5,003,246 issued Mar. 26, 1991, which is incorporated herein by reference, also provides a common drain configuration. There are at least four problems with this circuit. Firstly, it has field effect transistors with common gates and output terminals connected across the full AC voltage. The result of this is that the voltage capability of the switch is limited by the gate-source voltage capability of these field effect transistors. Another problem is that the switch does not respond immediately to activating control signals, but rather waits for the next zero crossing before turning on. The third problem has to do with energy efficiency. To be energy-efficient, the control resistor R1 must have a high resistance value because it is loaded with almost the entire AC voltage. However, a consequence of this resistor having a high value is that the speed of response of the switch is slowed down considerably due to the gate capacitance of the field effect transistor TP2. One could, of course, speed up the switch by lowering the resistance of the resistor R1, but that would result in increased power dissipation in the resistor due to the high-voltage AC. Finally, it should be noted that the switch never really turns fully off because of current flowing through the control resistor R1 when the switch is in the off-state.